The intervertebral disc is composed of a thick outer ring of cartilage (annulus) and an inner gel-like substance (nucleus pulposus). The annulus contains collagen fibers that form concentric lamellae that surround the nucleus and insert into the endplates of the adjacent vertebral bodies. The nucleus pulposus comprises proteoglycans entrapped by a network of collagen and elastin fibers which has the capacity to bind water. When healthy, the intervertebral disc keeps the spine flexible and serves as a shock absorber by allowing the body to accept and dissipate loads across multiple levels in the spine.
Over time, the nucleus pulposus becomes less fluid and more viscous as a result of age, normal wear and tear, and damage caused from an injury. The proteoglycan and water from within the nucleus decreases which in turn results in the nucleus drying out and becoming smaller and compressed. Additionally, the annulus tends to thicken, desiccate, and become more rigid, lessening its ability to elastically deform under load and making it susceptible to disc fissures.
A fissure occurs when the fibrous components of the annulus become separated in particular areas, creating a tear within the annulus. The most common type of fissure is a radial fissure in which the tear is perpendicular to the direction of the fibers. A fissure associated with disc herniation generally falls into three types of categories: 1) contained disc herniation (also known as contained disc protrusion); 2) extruded disc herniation; and 3) sequestered disc herniation (also known as a free fragment.)
In a contained herniation, a portion of the disc protrudes or bulges from a normal boundary of the disc but does not breach the outer annulus fibrosis. In an extruded herniation, the annulus is disrupted and a segment of the nucleus protrudes/extrudes from the disc. However, in this condition, the nucleus within the disc remains contiguous with the extruded fragment. With a sequestered disc herniation, a nucleus fragment separates from the nucleus and disc.
As the posterior and posterolateral portions of the annulus are most susceptible to herniation, in many instances, the nucleus pulposus progresses into the fissure from the nucleus in a posteriorly or posterolateral direction. Additionally, biochemicals contained within the nucleus pulposus may escape through the annulus causing inflammation and irritating adjacent nerves. Symptoms of a herniated disc generally include sharp back or neck pain which radiates into the extremities, numbness, muscle weakness, and in late stages, paralysis, muscle atrophy and bladder and bowel incontinence.
Conservative therapy is the first line of treating a herniated disc which includes bed rest, medications to reduce inflammation and pain, physical therapy, patient education on proper body mechanics and weight control.
However, if conservative therapy offers no improvement then surgery is recommended. Open discectomy is the most common surgical treatment for ruptured or herniated discs. The procedure involves an incision in the skin over the spine to remove the herniated disc material so it no longer presses on the nerves and spinal cord. Before the disc material is removed, some of the bone from the affected vertebra may be removed using a laminotomy or laminectomy to allow the surgeon to better see the area.
However, minimally invasive techniques have been rapidly replacing open surgery in treating herniated discs. Minimally invasive surgery utilizes small skin incisions, thereby minimizing the damaging effects of large muscle retraction and offering rapid recovery, less post-operative pain and small incisional scars. Examples of well-known minimally invasive techniques are provided below.
Microdiscectomy employs a surgical microscope and microsurgical techniques to view the disc and nerves. The magnified view makes it possible for the surgeon to remove only the herniated disc material which is “pinching” one or more spinal nerve root through a smaller incision, thus causing less damage to surrounding tissue.
Percutaneous discectomy uses a needle-like device which enters the disc space posterolaterally to remove the herniated disc material in a piecemeal fashion. Examples of such devices are described in U.S. Pat. No. 4,678,459. Using suction to pull in disc material, these cutting devices use a slide-like motion to slice the tissue which is then aspirated to a collection bottle. However, as illustrated in FIGS. 1A and 1B, these prior art devices are typically rigid and therefore, only able to access the center of the nucleus and remove material along a linear path from the access point to the center of the nucleus (1). As a result, the tissue removal is not performed at the site of the injury and thereby has limited effectiveness. Furthermore, the rigid devices are typically able to treat only the L4-L5 disc and not the L5-S1 disc that is commonly the source of patient discomfort. The L5-S1 disc is the lowest disc on the spine and because it is below the iliac crest, it is accessible by approaching it from an angle of approximately 30 degrees above the plane of the disc. A rigid device cannot access the disc unless it can approach the disc from within the same plane as the disc.
Endoscopic discectomy inserts an endoscopic probe between the vertebrae and into the herniated disc space through the skin of the back using an x-ray video image for guidance. Surgical attachments (cutters, lasers, and the like) are then sent down the hollow center of the probe to remove a portion of the offending disc. However, this direct approach to the disc herniation results in further injury to the already weakened disc annulus, thereby increasing the likelihood of subsequent herniations. Sometimes, the surgical attachments can be used to push the bulging disc back into place and for the removal of disc fragments and small bone spurs. The surgeon introduces the endoscope through a relatively large, (approximately 10 mm or greater), incision into the skin above the spine, then locates the nerve and disc using direct visualization.
Chemonucleolysis involves the injection of chymopapain or other nucleus dissolving substance into the disc to partially dissolve the nucleus to alleviate disc herniation. Chymopapain is an enzyme that works by depolymerizing the proteoglycan and glycoprotein molecules in the nucleus pulposus. These large molecules are responsible for water retention and turgidity. When exposed to chymopapain, the water content within the disc decreases resulting in shrinkage, thereby causing a reduction in disc height and girth.
Nucleoplasty involves the percutaneous removal of disc material by using a low-temperature resister probe to disintegrate and evacuate disc material, followed by thermal treatment of adjacent residual disc material. The procedure combines disc removal and thermal coagulation to putatively decompress a contained herniated disc. A posterolateral approach is guided by fluoroscopy and a discogram may take place at this time to confirm location. Taking care not to contact the anterior annulus, the nucleus pulposus is first ablated with radiofrequency waves as the wand is advanced causing a molecular dissociation process converting tissue into gas which putatively escapes through the needle. As the wand is withdrawn, coagulation takes place thermally treating the channel, which leads to a denaturing of nerve fibers adjacent to the channel within the nucleus pulposus.
Intradiscal electrothermal therapy involves the percutaneous insertion of a specially designed thermal resistance probe followed by controlled heating of the intervertebral disc. This may result in limited annulus contraction and coagulation of nerve tissue and reduction in pain. A needle is inserted posterolaterally into the disc, generally from the patient's less painful side. A cannula with a flexible heating tip is threaded circumferentially into the disc through the nucleus pulposus to the pathologic area of the annulus.
Percutaneous Laser Discectomy involves directing a laser to the target tissue area which absorbs the laser light and converts it to heat. When the temperature reaches 100° C., tissue vaporizes and ablation takes places. As a small amount of nucleus pulposus is vaporized, intradiscal pressure decreases, allowing the disc to return to its normal state.
The above methods typically employ a posterior lateral approach for accessing the nucleus. This approach relies on a rigid introducer to pierce through the annulus and access the nucleus pulposus. Generally only the center or anterior portion of the nucleus is accessible with these methods because of the entry angle and density of the nucleus. However, most herniations typically occur in the posterior portion because the posterior wall of the annulus is thinner than the anterior wall. Therefore, the above methods are unable to access and treat most herniations at the specific site of injury.
Additionally, the above methods are based on the concept in which the intervertebral disc acts as a closed hydraulic system. According to this theory, the nucleus pulposus contains a large amount of water and is surrounded by the inelastic annulus fibrosis. Therefore, disc pressure decreases by removing nucleus pulposus material from the center of the nucleus which causes the herniated disc material to recede toward the center of the disc. As such, the above methods are designed to merely remove a portion of the nucleus pulposus within the center of the nucleus but do not specifically remove material from within the site of injury.
In light of the foregoing discussion, there is a need for a device that allows for active and directional navigation within the confines of the disc to directly access the injured disc material at the site of herniation. In particular, there is a need for a device that can navigate within the nucleus pulposus and excise material directly from the site of herniation within the fissure.